Mimo system with multiple spatial multiplexing modes

ABSTRACT

A MIMO system supports multiple spatial multiplexing modes for improved performance and greater flexibility. These modes may include (1) a single-user steered mode that transmits multiple data streams on orthogonal spatial channels to a single receiver, (2) a single-user non-steered mode that transmits multiple data streams from multiple antennas to a single receiver without spatial processing at a transmitter, (3) a multi-user steered mode that transmits multiple data streams simultaneously to multiple receivers with spatial processing at a transmitter, and (4) a multi-user non-steered mode that transmits multiple data streams from multiple antennas (co-located or non co-located) without spatial processing at the transmitter(s) to receiver(s) having multiple antennas. For each set of user terminal(s) selected for data transmission on the downlink and/or uplink, a spatial multiplexing mode is selected for the user terminal set from among the multiple spatial multiplexing modes supported by the system.

CLAIM OF PRIORITY

This application for patent is a divisional application of, and claimsthe benefit of priority from, U.S. patent application Ser. No.10/693,429, entitled “MIMO System with Multiple Spatial MultiplexingModes” and filed on Oct. 23, 2003, which claims the benefit of priorityfrom U.S. Provisional Patent Application Ser. No. 60/421,309, entitled“MIMO WLAN System” and filed Oct. 25, 2002, both of which are assignedto the assignee of this application for patent, and are fullyincorporated herein by reference for all purposes.

BACKGROUND

1. Field

The present invention relates generally to communication, and morespecifically to a multiple-input multiple-output (MIMO) communicationsystem with multiple transmission modes.

2. Background

A MIMO system employs multiple (N_(T)) transmit antennas and multiple(N_(R)) receive antennas for data transmission and is denoted as an(N_(T), N_(R)) system. A MIMO channel formed by the N_(T) transmit andN_(R) receive antennas may be decomposed into N_(S) spatial channels,where N_(S)≦min {N_(T), N_(R)}. The N_(S) spatial channels may be usedto transmit N_(S) independent data streams to achieve greater overallthroughput. In general, spatial processing may or may not be performedat a transmitter and is normally performed at a receiver tosimultaneously transmit and recover multiple data streams.

A conventional MIMO system typically uses a specific transmission schemeto simultaneously transmit multiple data streams. This transmissionscheme may be selected based on a trade-off of various factors such asthe requirements of the system, the amount of feedback from the receiverto the transmitter, the capabilities of the transmitter and receiver,and so on. The transmitter, receiver, and system are then designed tosupport and operate in accordance with the selected transmission scheme.This transmission scheme typically has favorable features as well asunfavorable ones, which can impact system performance.

There is therefore a need in the art for a MIMO system capable ofachieving improved performance.

SUMMARY

A MIMO system that supports multiple spatial multiplexing modes forimproved performance and greater flexibility is described herein.Spatial multiplexing refers to the transmission of multiple data streamssimultaneously via multiple spatial channels of a MIMO channel. Themultiple spatial multiplexing modes may include (1) a single-usersteered mode that transmits multiple data streams on orthogonal spatialchannels to a single receiver, (2) a single-user non-steered mode thattransmits multiple data streams from multiple antennas to a singlereceiver without spatial processing at a transmitter, (3) a multi-usersteered mode that transmits multiple data streams simultaneously tomultiple receivers with spatial processing at a transmitter, and (4) amulti-user non-steered mode that transmits multiple data streams frommultiple antennas (co-located or non co-located) without spatialprocessing at the transmitter(s) to receiver(s) having multipleantennas.

A set of at least one user terminal is selected for data transmission onthe downlink and/or uplink. A spatial multiplexing mode is selected forthe user terminal set from among the multiple spatial multiplexing modessupported by the system. Multiple rates are also selected for multipledata streams to be transmitted via multiple spatial channels of a MIMOchannel for the user terminal set. The user terminal set is scheduledfor data transmission on the downlink and/or uplink with the selectedrates and the selected spatial multiplexing mode. Thereafter, multipledata streams are processed (e.g., coded, interleaved, and modulated) inaccordance with the selected rates and further spatially processed inaccordance with the selected spatial multiplexing mode for transmissionvia multiple spatial channels.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a multiple-access MIMO system;

FIG. 2 shows a frame and channel structure for the MIMO system;

FIG. 3 shows an access point and two user terminals in the MIMO system;

FIG. 4 shows a transmit (TX) data processor at the access point;

FIG. 5 shows a TX spatial processor and modulators at the access point;

FIG. 6 shows demodulators and a receive (RX) spatial processor at amulti-antenna user terminal;

FIG. 7 shows an RX data processor at the multi-antenna user terminal;

FIG. 8 shows an RX spatial processor and an RX data processor thatimplement a successive interference cancellation (SIC) technique;

FIG. 9 shows the transmit/receive chains at the access point and userterminal;

FIG. 10 shows a closed-loop rate control mechanism;

FIG. 11 shows a controller and a scheduler for scheduling userterminals;

FIG. 12 shows a process for scheduling user terminals for datatransmission;

FIG. 13 shows a process for transmitting data on the downlink; and

FIG. 14 shows a process for receiving data on the uplink.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

A MIMO system may utilize a single carrier or multiple carriers for datatransmission. Multiple carriers may be provided by orthogonal frequencydivision multiplexing (OFDM), other multi-carrier modulation techniques,or some other constructs. OFDM effectively partitions the overall systembandwidth into multiple (N_(F)) orthogonal subbands, which are alsocommonly referred to as tones, bins, carriers, and frequency channels.With OFDM, each subband is associated with a respective carrier that maybe modulated with data. The following description is for a MIMO systemthat utilizes OFDM. However, the concepts described herein are equallyapplicable for a single carrier MIMO system.

The MIMO system supports multiple spatial multiplexing modes forimproved performance and greater flexibility. Table 1 lists thesupported spatial multiplexing modes and their short descriptions.

TABLE 1 Spatial Multiplexing Mode Description Single-User Multiple datastreams are transmitted on orthogonal spatial Steered channels to asingle receiver. Single-User Multiple data streams are transmitted frommultiple Non-Steered antennas to a single receiver without spatialprocessing at a transmitter. Multi-User Multiple data streams aretransmitted simultaneously (1) Steered from a single transmitter tomultiple receivers or (2) from multiple transmitters to a singlereceiver, both with spatial processing at the transmitter(s). Multi-UserMultiple data streams are transmitted simultaneously (1) Non-Steeredfrom multiple transmitters to a single receiver or (2) from a singletransmitter to multiple receivers, both without spatial processing atthe transmitter(s).The MIMO system may also support other and/or different spatialmultiplexing modes, and this is within the scope of the invention.

Each spatial multiplexing mode has different capabilities andrequirements. The steered spatial multiplexing modes can typicallyachieve better performance but can only be used if the transmitter hassufficient channel state information to orthogonalize the spatialchannels via decomposition or some other technique, as described below.The non-steered spatial multiplexing modes require very littleinformation to simultaneously transmit multiple data streams, butperformance may not be quite as good as the steered spatial multiplexingmodes. A suitable spatial multiplexing mode may be selected for usedepending on the available channel state information, the capabilitiesof the transmitter and receiver, system requirements, and so on. Each ofthese spatial multiplexing modes is described below.

Single-User Steered Spatial Multiplexing Mode

A frequency-selective MIMO channel formed by N_(T) transmit antennas andN_(R) receive antennas may be characterized by N_(F) frequency-domainchannel response matrices H(k), for k=1 . . . N_(F), each withdimensions of N_(R)×N_(T). The channel response matrix for each subbandmay be expressed as:

$\begin{matrix}{{{\underset{\_}{H}(k)} = \begin{bmatrix}{h_{1,1}(k)} & {h_{1,2}(k)} & \ldots & {h_{1,N_{T}}(k)} \\{h_{2,1}(k)} & {h_{2,2}(k)} & \ldots & {h_{2,N_{T}}(k)} \\\vdots & \vdots & \ddots & \vdots \\{h_{N_{R},1}(k)} & {h_{N_{R},2}(k)} & \ldots & {h_{N_{R},N_{T}}(k)}\end{bmatrix}},} & {{Eq}.\mspace{14mu} (1)}\end{matrix}$

-   where entry h_(i,j)(k) for i=1 . . . N_(R), j=1 . . . N_(T), and k=1    . . . N_(F), is the coupling (i.e., complex gain) between transmit    antenna j and receive antenna i for subband k.

The channel response matrix H(k) for each subband may be “diagonalized”to obtain N_(S) eigenmodes for that subband. This diagonalization may beachieved by performing either singular value decomposition of thechannel response matrix H(k) or eigenvalue decomposition of acorrelation matrix of H(k), which is R(k)=H^(H)(k)H(k), where “^(H)”denotes the conjugate transpose.

The singular value decomposition of the channel response matrix H(k) foreach subband may be expressed as:

H(k)=U(k)Σ(k)V ^(H)(k),  Eq. (2)

where

-   -   U(k) is an (N_(R)×N_(R)) unitary matrix of left eigenvectors of        H(k);    -   Σ(k) is an (N_(R)×N_(T)) diagonal matrix of singular values of        H(k); and    -   V(k) is an (N_(T)×N_(T)) unitary matrix of right eigenvectors of        H(k).        A unitary matrix M is characterized by the property M^(H)M=I,        where I is the identity matrix. The columns of a unitary matrix        are orthogonal to one another.

The eigenvalue decomposition of the correlation matrix of H(k) for eachsubband may be expressed as:

R(k)=H ^(H)(k)H(k)=V(k)∀(k)V ^(H)(k),  Eq. (3)

-   where ∀(k) is an (N_(T)×N_(T)) diagonal matrix of eigenvalues of    R(k). As shown in equations (2) and (3), the columns of V(k) are    eigenvectors of R(k) as well as right eigenvectors of H(k).

Singular value decomposition and eigenvalue decomposition are describedby Gilbert Strang in a book entitled “Linear Algebra and ItsApplications,” Second Edition, Academic Press, 1980. The single-usersteered spatial multiplexing mode may be implemented with eithersingular value decomposition or eigenvalue decomposition. For clarity,singular value decomposition is used for the following description.

The right eigenvectors of H(k) are also referred to as “steering”vectors and may be used for spatial processing by a transmitter totransmit data on the Ns eigenmodes of H(k). The left eigenvectors ofH(k) may be used for spatial processing by a receiver to recover thedata transmitted on the N_(S) eigenmodes. The eigenmodes may be viewedas orthogonal spatial channels obtained through decomposition. Thediagonal matrix Σ(k) contains non-negative real values along thediagonal and zeros elsewhere. These diagonal entries are referred to asthe singular values of H(k) and represent the channel gains for theN_(S) eigenmodes of H(k). The singular values of H(k), {σ₁(k) σ₂(k) . .. σ_(N) _(S) (k)}, are also the square roots of the eigenvalues of R(k),{λ₁(k) λ₂(k) . . . λ_(N) _(S) (k)}, where σ_(i)(k)=√{square root over(λ_(i)(k))}. Singular value decomposition may be performed independentlyon the channel response matrix H(k) for each of the N_(F) subbands todetermine the N_(S) eigenmodes for that subband.

For each subband, the singular values in the matrix Σ(k) may be orderedfrom largest to smallest, and the eigenvectors in the matrices V(k) andU(k) may be ordered correspondingly. A “wideband” eigenmode may bedefined as the set of same-order eigenmodes of all N_(F) subbands afterthe ordering (i.e., wideband eigenmode m includes eigenmode m of allsubbands). In general, all or fewer than N_(F) subbands may be used fortransmission, with the unused subbands being filled with signal valuesof zero. For simplicity, the following description assumes that allN_(F) subbands are used for transmission.

The single-user steered spatial multiplexing mode (or simply, the“single-user steered mode”) transmits N_(S) data symbol streams on theN_(S) eigenmodes of the MIMO channel. This requires spatial processingby both the transmitter and the receiver.

The spatial processing at the transmitter for each subband for thesingle-user steered mode may be expressed as:

x _(su-s)(k)=V(k)s(k),  Eq. (4)

where

-   -   s(k) is an (N_(T)×1) vector with N_(S) non-zero entries for        N_(S) data symbols to be transmitted on the N_(S) eigenmodes for        subband k; and    -   x_(su-s)(k) is an (N_(T)×1) vector with N_(T) entries for N_(T)        transmit symbols to be sent from the N_(T) transmit antennas for        subband k.        The N_(S) entries of s(k) can represent N_(S) data symbol        streams and the remaining entries of s(k), if any, are filled        with zeros.

The received symbols obtained by the receiver for each subband may beexpressed as:

r _(su-s)(k)=H(k)x _(su-s)(k)+n(k)=H(k)V(k)s(k)+n(k),  Eq. (5)

where

-   -   r_(su-s)(k) is an (N_(R)×1) vector with N_(R) entries for N_(R)        received symbols obtained via the N_(R) receive antennas for        subband k; and    -   n(k) is a noise vector for subband k.

The spatial processing at the receiver to recover the data vector s(k)for each subband may be expressed as:

$\begin{matrix}{\begin{matrix}{{{{\underset{\_}{\hat{s}}}_{{su} - s}(k)} = {{{\underset{\_}{\Sigma}}^{- 1}(k)}{{\underset{\_}{U}}^{H}(k)}{{\underset{\_}{r}}_{{su} - s}(k)}}},} \\{{= {{{\underset{\_}{\Sigma}}^{- 1}(k)}{{\underset{\_}{U}}^{H}(k)}\left( {{{\underset{\_}{H}(k)}{\underset{\_}{V}(k)}{\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}} \right)}},} \\{{= {{{\underset{\_}{\Sigma}}^{- 1}(k)}{{\underset{\_}{U}}^{H}(k)}\left( {{{\underset{\_}{U}(k)}{\underset{\_}{\Sigma}(k)}{{\underset{\_}{V}}^{H}(k)}{\underset{\_}{V}(k)}{\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}} \right)}},} \\{{= {{\underset{\_}{s}(k)} + {{\underset{\_}{n}}_{{su} - s}(k)}}},}\end{matrix}{{{{\underset{\_}{\overset{\sim}{s}}}_{{su} - s}(k)} = {{{{\underset{\_}{U}}^{H}(k)}{{\underset{\_}{r}}_{{su} - s}(k)}\mspace{14mu} {and}\mspace{14mu} {{\underset{\_}{\hat{s}}}_{{su} - s}(k)}} = {{{\underset{\_}{\Sigma}}^{- 1}(k)}{{\underset{\_}{\overset{\sim}{s}}}_{{su} - s}(k)}}}},}} & {{Eq}.\mspace{14mu} (6)}\end{matrix}$

orwhere

-   -   {tilde over (s)}_(su-s)(k) is an (N_(T)×1) vector with N_(S)        detected data symbols for subband k;    -   ŝ_(su-s)(k) is an (N_(T)×1) vector with N_(S) recovered data        symbols for subband k; and    -   n_(su-s)(k) is a vector of post-processed noise for subband k.        The vector {tilde over (s)}_(su-s)(k) is an unnormalized        estimate of the data vector s(k), and the vector ŝ_(su-s)(k) is        a normalized estimate of s(k). The multiplication by Σ⁻¹(k) in        equation (6) accounts for the (possibly different) gains of the        N_(S) spatial channels and normalizes the output of the receiver        spatial processing so that recovered data symbols with the        proper magnitude are provided to a subsequent processing unit.

For the single-user steered mode, the matrix F_(su-s)(k) of steeringvectors used by the transmitter for each subband may be expressed as:

F _(su-s)(k)=V(k)  Eq. (7)

The spatial filter matrix used by the receiver for each subband may beexpressed as:

M _(su-s)(k)=U ^(H)(k).  Eq. (8)

The single-user steered mode may be used if the transmitter has channelstate information for either the channel response matrix H(k) or thematrix V(k) of right eigenvectors of H(k), for k=1 . . . N_(F). Thetransmitter can estimate H(k) or V(k) for each subband based on a pilottransmitted by the receiver, as described below, or may be provided withthis information by the receiver via a feedback channel. The receivercan typically obtain H(k) or U^(H)(k) for each subband based on a pilottransmitted by the transmitter. Equation (6) indicates that the N_(S)data symbol streams s(k), distorted only by post-processed channel noisen_(su-s)(k), may be obtained for the single-user steered mode with theproper spatial processing at both the transmitter and the receiver.

The signal-to-noise-and-interference ratio (SNR) for the single-usersteered mode may be expressed as:

$\begin{matrix}{{{\gamma_{{{su} - s},m}(k)} = \frac{{P_{m}(k)}{\lambda_{m}(k)}}{\sigma^{2}}},{m = {1\mspace{14mu} \ldots \mspace{14mu} N_{S}}},} & {{Eq}.\mspace{14mu} (9)}\end{matrix}$

where

-   -   P_(m)(k) is the transmit power used for the data symbol        transmitted on subband k of wideband eigenmode m;    -   λ_(m)(k) is the eigenvalue for subband k of wideband eigenmode        m, which is the m-th diagonal element of ∀(k); and    -   γ_(su-s,m)(k) is the SNR for subband k of wideband eigenmode m.

Single-User Non-Steered Spatial Multiplexing Mode

The single-user non-steered spatial multiplexing mode (or simply, the“single-user non-steered mode”) may be used if the transmitter does havenot sufficient channel state information or if the single-user steeredmode cannot be supported for any reasons. The single-user non-steeredmode transmits N_(S) data symbol streams from N_(T) transmit antennaswithout any spatial processing at the transmitter. For the single-usernon-steered mode, the matrix F_(ns)(k) of steering vectors used by thetransmitter for each subband may be expressed as:

F _(ns)(k)=I  Eq. (10)

The spatial processing at the transmitter for each subband may beexpressed as:

x _(ns)(k)=s(k),  Eq. (11)

-   where x_(ns)(k) is the transmit symbol vector for the single-user    non-steered mode. A “wideband” spatial channel for this mode may be    defined as the spatial channel corresponding to a given transmit    antenna (i.e., wideband spatial channel m for the single-user    non-steered mode includes all subbands of transmit antenna m).

The received symbols obtained by the receiver for each subband may beexpressed as:

r _(ns)(k)=H(k)x _(ns)(k)+n(k)=H(k)s(k)+n(k).  Eq (12)

The receiver can recover the data vector s(k) using various receiverprocessing techniques such as a channel correlation matrix inversion(CCMI) technique (which is also commonly referred to as a zero-forcingtechnique), a minimum mean square error (MMSE) technique, a decisionfeedback equalizer (DFE), a successive interference cancellation (SIC)technique, and so on.

CCMI Spatial Processing

The receiver can use the CCMI technique to separate out the data symbolstreams. A CCMI receiver utilizes a spatial filter having a response ofM_(ccmi)(k), for k=1 . . . N_(F), which can be expressed as:

M _(ccmi)(k)=[H ^(H)(k)H(k)]⁻¹ H ^(H)(k)=R ⁻¹(k)H ^(H)(k)  Eq. (13)

The spatial processing by the CCMI receiver for the single-usernon-steered mode may be expressed as:

$\begin{matrix}\begin{matrix}{{{{\underset{\_}{\hat{s}}}_{ccmi}(k)} = {{{\underset{\_}{M}}_{ccmi}(k)}{{\underset{\_}{r}}_{n\; s}(k)}}},} \\{{= {{{\underset{\_}{R}}^{- 1}(k)}{{\underset{\_}{H}}^{H}(k)}\left( {{{\underset{\_}{H}(k)}{\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}} \right)}},} \\{{= {{\underset{\_}{s}(k)} + {{\underset{\_}{n}}_{ccmi}(k)}}},}\end{matrix} & {{Eq}.\mspace{14mu} (14)}\end{matrix}$

where

-   -   ŝ_(ccmi)(k) is an (N_(T)×1) vector with N_(S) recovered data        symbols for subband k; and    -   n_(ccmi)(k)=M_(ccmi)(k)n(k) is the CCMI filtered noise for        subband k.

An autocovariance matrix φ_(ccmi)(k) of the CCMI filtered noise for eachsubband may be expressed as:

$\begin{matrix}\begin{matrix}{{{{\underset{\_}{\phi}}_{ccmi}(k)} = {E\left\lbrack {{{\underset{\_}{n}}_{ccmi}(k)}{{\underset{\_}{n}}_{ccmi}^{H}(k)}} \right\rbrack}},} \\{{= {{{\underset{\_}{M}}_{ccmi}(k)}{{\underset{\_}{\phi}}_{nn}(k)}{{\underset{\_}{M}}_{ccmi}^{H}(k)}}},} \\{{= {\sigma^{2}{{\underset{\_}{R}}^{- 1}(k)}}},}\end{matrix} & {{Eq}.\mspace{14mu} (15)}\end{matrix}$

-   where E[x] is the expected value of x. The last equality in    equation (15) assumes that the noise n(k) is additive white Gaussian    noise (AWGN) with zero mean, a variance of σ², and an autocovariance    matrix of φ_(nn)(k)=E[n(k)n^(H)(k)]=σ²I. In this case, the SNR for    the CCMI receiver may be expressed as:

$\begin{matrix}{{{\gamma_{{ccmi},m}(k)} = \frac{P_{m}(k)}{{r_{m\; m}(k)}\sigma^{2}}},{m = {1\mspace{14mu} \ldots \mspace{14mu} N_{S}}},} & {{Eq}.\mspace{14mu} (16)}\end{matrix}$

where

-   -   P_(m)(k) is the transmit power used for the data symbol        transmitted on subband k of wideband spatial channel m;    -   r_(mm)(k) is the m-th diagonal element of R(k) for subband k;        and    -   γ_(ccmi,m)(k) is the SNR for subband k of wideband spatial        channel m.        Due to the structure of R(k), the CCMI technique may amplify the        noise.

MMSE Spatial Processing

The receiver can use the MMSE technique to suppress crosstalk betweenthe data symbol streams and maximize the SNRs of the recovered datasymbol streams. An MMSE receiver utilizes a spatial filter having aresponse of M_(mmse)(k), for k=1 . . . N_(F), which is derived such thatthe mean square error between the estimated data vector from the spatialfilter and the data vector s(k) is minimized. This MMSE criterion may beexpressed as:

$\begin{matrix}{\min\limits_{({{\underset{\_}{M}}_{mmse}{(k)}})}{{E\begin{bmatrix}\left( {{{{\underset{\_}{M}}_{mmse}(k)}{{\underset{\_}{r}}_{n\; s}(k)}} - {\underset{\_}{s}(k)}} \right)^{H} \\\left( {{{{\underset{\_}{M}}_{mmse}(k)}{{\underset{\_}{r}}_{n\; s}(k)}} - {\underset{\_}{s}(k)}} \right)\end{bmatrix}}.}} & {{Eq}.\mspace{14mu} (17)}\end{matrix}$

The solution to the optimization problem posed in equation (17) may beobtained in various manners. In one exemplary method, the MMSE spatialfilter matrix M_(mmse)(k) for each subband may be expressed as:

$\begin{matrix}\begin{matrix}{{{{\underset{\_}{M}}_{mmse}(k)} = {{{\underset{\_}{H}}^{H}(k)}\left\lbrack {{{\underset{\_}{H}(k)}{{\underset{\_}{H}}^{H}(k)}} + {{\underset{\_}{\phi}}_{nn}(k)}} \right\rbrack}^{- 1}},} \\{= {{{{\underset{\_}{H}}^{H}(k)}\left\lbrack {{{\underset{\_}{H}(k)}{{\underset{\_}{H}}^{H}(k)}} + {\sigma^{2}\underset{\_}{I}}} \right\rbrack}^{- 1}.}}\end{matrix} & {{Eq}.\mspace{14mu} (18)}\end{matrix}$

The second equality in equation (18) assumes that the noise vector n(k)is AWGN with zero mean and variance of σ².

The spatial processing by the MMSE receiver for the single-usernon-steered mode is composed of two steps. In the first step, the MMSEreceiver multiplies the vector r_(ns)(k) for the N_(R) received symbolstreams with the MMSE spatial filter matrix M_(mmse)(k) to obtain avector {tilde over (s)}_(mmse)(k) for N_(S) detected symbol streams, asfollows:

$\begin{matrix}\begin{matrix}{{{{\overset{\sim}{\underset{\_}{s}}}_{mmse}(k)} = {{{\underset{\_}{M}}_{mmse}(k)}{{\underset{\_}{r}}_{n\; s}(k)}}},} \\{{= {{{\underset{\_}{M}}_{mmse}(k)}\left( {{{\underset{\_}{H}(k)}{\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}} \right)}},} \\{{= {{{\underset{\_}{Q}(k)}{\underset{\_}{s}(k)}} + {{\underset{\_}{n}}_{mmse}(k)}}},}\end{matrix} & {{Eq}.\mspace{14mu} (19)}\end{matrix}$

where

-   -   n_(mmse)(k)=M_(mmse)(k)n(k) is the MMSE filtered noise and    -   Q(k)=M_(mmse)(k)H(k)        The N_(S) detected symbol streams are unnormalized estimates of        the N_(S) data symbol streams.

In the second step, the MMSE receiver multiplies the vector {tilde over(s)}_(mmse)(k) with a scaling matrix D_(mmse) ⁻¹(k) to obtain a vectorŝ_(mmse)(k) for the N_(S) recovered data symbol streams, as follows:

ŝ _(mmse)(k)=D _(mmse) ⁻¹(k){tilde over (s)} _(mmse)(k),  Eq. (20)

-   where D_(mmse)(k) is a diagonal matrix whose diagonal elements are    the diagonal elements of Q(k), i.e., D_(mmse)(k)=diag [Q(k)]. The    N_(S) recovered data symbol streams are normalized estimates of the    N_(S) data symbol streams.

Using the matrix inverse identity, the matrix Q(k) can be rewritten as:

$\begin{matrix}\begin{matrix}{{{\underset{\_}{Q}(k)} = {{{\underset{\_}{H}}^{H}(k)}{{\underset{\_}{\phi}}_{nn}^{- 1}(k)}{{\underset{\_}{H}(k)}\left\lbrack {{{{\underset{\_}{H}}^{H}(k)}{{\underset{\_}{\phi}}_{nn}^{- 1}(k)}{\underset{\_}{H}(k)}} + \underset{\_}{I}} \right\rbrack}^{- 1}}},} \\{= {{{\underset{\_}{H}}^{H}(k)}{{{\underset{\_}{H}(k)}\left\lbrack {{{{\underset{\_}{H}}^{H}(k)}{\underset{\_}{H}(k)}} + {\sigma^{2}\underset{\_}{I}}} \right\rbrack}^{- 1}.}}}\end{matrix} & {{Eq}.\mspace{14mu} (21)}\end{matrix}$

The second equality in equation (21) assumes that the noise is AWGN withzero mean and variance of σ².

The SNR for the MMSE receiver may be expressed as:

$\begin{matrix}{{{\gamma_{{mmse},m}(k)} = {\frac{q_{m\; m}(k)}{1 - {q_{m\; m}(k)}}{P_{m}(k)}}},{m = {1\mspace{14mu} \ldots \mspace{14mu} N_{S}}},} & {{Eq}.\mspace{14mu} (22)}\end{matrix}$

where

-   -   q_(mm)(k) is the m-th diagonal element of Q(k) for subband k;        and    -   γ_(mmse,m)(k) is the SNR for subband k of wideband spatial        channel m.

Successive Interference Cancellation Receiver Processing

The receiver can process the N_(R) received symbol streams using the SICtechnique to recover the N_(S) data symbol streams. For the SICtechnique, the receiver initially performs spatial processing on theN_(R) received symbol streams (e.g., using CCMI, MMSE, or some othertechnique) and obtains one recovered data symbol stream. The receiverfurther processes (e.g., demodulates, deinterleaves, and decodes) thisrecovered data symbol stream to obtain a decoded data stream. Thereceiver then estimates the interference this stream causes to the otherN_(S)−1 data symbol streams and cancels the estimated interference fromthe N_(R) received symbol streams to obtain N_(R) modified symbolstreams. The receiver then repeats the same processing on the N_(R)modified symbol streams to recover another data symbol stream. For a SICreceiver, the input (i.e., received or modified) symbol streams forstage l, where l=1 . . . N_(S), may be expressed as:

r _(sic) ^(l)(k)=H ^(l)(k)x _(ns) ^(l)(k)+n(k)=H ^(l)(k)s^(l)(k)+n(k),  Eq. (23)

where

-   -   r_(sic) ^(l)(k) is a vector of N_(R) modified symbols for        subband k in stage l, and    -   r_(sic) ^(l)(k)=r_(ns)(k) for the first stage;    -   s^(l)(k) is a vector of (N_(T)−l+1) data symbols not yet        recovered for subband k in stage l; and    -   H^(l)(k) is an N_(R)×(N_(T)−l+1) reduced channel response matrix        for subband k in stage l.

Equation (23) assumes that the data symbol streams recovered in the(l−1) prior stages are canceled. The dimensionality of the channelresponse matrix H(k) successively reduces by one column for each stageas a data symbol stream is recovered and canceled. For stage l, thereduced channel response matrix H^(l)(k) is obtained by removing (l−1)columns in the original matrix H(k) corresponding to the (l−1) datasymbol streams previously recovered, i.e., H^(l)(k)=[h_(j) _(l) (k)h_(j)_(l−1) (k) . . . h_(jN) _(T) (k)], where h_(j) _(n) (k) is an N_(R)×1vector for the channel response between transmit antenna j_(n) and theN_(R) receive antennas. For stage l, the (l−1) data symbol streamsrecovered in the prior stages are given indices of {j₁ j₂ . . .j_(l−1)}, and the (N_(T)−l+1) data symbol streams not yet recovered aregiven indices of {j_(l) j_(l+1) . . . j_(N) _(T) }.

For stage l, the SIC receiver derives a spatial filter matrix M_(sic)^(l)(k), for k=1 . . . N_(F), based on the reduced channel responsematrix H^(l)(k) (instead of the original matrix H(k)) using the CCMItechnique as shown in equation (13), the MMSE technique as shown inequation (18), or some other technique. The matrix M_(sic) ^(l)(k) hasdimensionality of (N_(T)−l+1)×N_(R). Since H^(l)(k) is different foreach stage, the spatial filter matrix M_(sic) ^(l)(k) is also differentfor each stage.

The SIC receiver multiplies the vector r_(sic) ^(l)(k) for the N_(R)modified symbol streams with the spatial filter matrix M_(sic) ^(l)(k)to obtain a vector {tilde over (s)}_(sic) ^(l)(k) for (N_(T)−l+1)detected symbol streams, as follows:

$\begin{matrix}\begin{matrix}{{{{\underset{\_}{\overset{\sim}{s}}}_{sic}^{l}(k)} = {{{\underset{\_}{M}}_{sic}^{l}(k)}{{\underset{\_}{r}}_{sic}^{l}(k)}}},} \\{{= {{{\underset{\_}{M}}_{sic}^{l}(k)}\left( {{{{\underset{\_}{H}}^{l}(k)}{{\underset{\_}{s}}^{l}(k)}} + {{\underset{\_}{n}}^{l}(k)}} \right)}},} \\{{= {{{{\underset{\_}{Q}}_{sic}^{l}(k)}{{\underset{\_}{s}}^{l}(k)}} + {{\underset{\_}{n}}_{sic}^{l}(k)}}},}\end{matrix} & {{Eq}.\mspace{14mu} (24)}\end{matrix}$

-   where n_(sic) ^(l)(k)=M_(sic) ^(l)(k)n^(l)(k) is the filtered noise    for subband k of stage l, n^(l)(k) is a reduced vector of n(k), and    Q_(sic) ^(l)(k)=M_(sic) ^(l)(k)H^(l)(k).    The SIC receiver then selects one of the detected symbol streams for    recovery. Since only one data symbol stream is recovered in each    stage, the SIC receiver can simply derive one (1×N_(R)) spatial    filter row vector m_(jl) ^(l)(k) for the data symbol stream {s_(jl)}    to be recovered in stage l. The row vector m_(jl) ^(l)(k) is one row    of the matrix M_(sic) ^(l)(k). In this case, the spatial processing    for stage l to recover the data symbol stream {s_(jl)} may be    expressed as:

{tilde over (s)} _(jl)(k)=m _(jl) ^(l)(k)r _(sic) ^(l)(k)=q _(jl)^(l)(k)s ^(l)(k)+m _(jl) ^(l)(k)n(k),  Eq. (25)

where q_(jl) ^(l)(k) is the row of Q_(sic) ^(l)(k) corresponding to datasymbol stream {s_(jl)}.

In any case, the receiver scales the detected symbol stream {{tilde over(s)}_(jl)} to obtain a recovered data symbol stream {ŝ_(jl)} and furtherprocesses (e.g., demodulates, deinterleaves, and decodes) the stream{ŝ_(jl)} to obtain a decoded data stream {{circumflex over (d)}_(jl)}.The receiver also forms an estimate of the interference this streamcauses to the other data symbol streams not yet recovered. To estimatethe interference, the receiver re-encodes, interleaves, and symbol mapsthe decoded data stream {{circumflex over (d)}_(jl)} in the same manneras performed at the transmitter and obtains a stream of “remodulated”symbols {j I}, which is an estimate of the data symbol stream justrecovered. The receiver then convolves the remodulated symbol streamwith each of N_(R) elements in the channel response vector h_(jl)(k) forstream {s_(jl)} to obtain N_(R) interference components i_(jl)(k) causedby this stream. The N_(R) interference components are then subtractedfrom the N_(R) modified symbol streams r_(sic) ^(l)(k) for stage l toobtain N_(R) modified symbol streams r_(sic) ^(l)(k) for the next stagel+1, i.e., r_(sic) ^(l+1)(k)=r_(sic) ^(l)(k)−i_(jl)(k). The modifiedsymbol streams r_(sic) ^(l+1)(k) represent the streams that would havebeen received if the data symbol stream {s_(jl)} had not beentransmitted (i.e., assuming that the interference cancellation waseffectively performed).

The SIC receiver processes the N_(R) received symbol streams in N_(S)successive stages. For each stage, the SIC receiver (1) performs spatialprocessing on either the N_(R) received symbol streams or the N_(R)modified symbol streams from the preceding stage to obtain one recovereddata symbol stream, (2) decodes this recovered data symbol stream toobtain a corresponding decoded data stream, (3) estimates and cancelsthe interference due to this stream, and (4) obtains N_(R) modifiedsymbol streams for the next stage. If the interference due to each datastream can be accurately estimated and canceled, then later recovereddata streams experience less interference and may be able to achievehigher SNRs.

For the SIC technique, the SNR of each recovered data symbol stream isdependent on (1) the spatial processing technique (e.g., CCMI or MMSE)used for each stage, (2) the specific stage in which the data symbolstream is recovered, and (3) the amount of interference due to datasymbol streams recovered in later stages. The SNR for the SIC receiverwith CCMI may be expressed as:

$\begin{matrix}{{{\gamma_{{{sic} - {ccmi}},m}(k)} = \frac{P_{m}(k)}{{r_{m\; m}^{l}(k)}\sigma^{2}}},{m = {1\mspace{14mu} \ldots \mspace{14mu} N_{S}}},} & {{Eq}.\mspace{14mu} (26)}\end{matrix}$

-   where r_(mm) ^(l)(k) is the m-th diagonal element of [R^(l)(k)]⁻¹    for subband k, where R^(l)(k)=[H^(l)(k)]^(H)H^(l)(k).

The SNR for the SIC receiver with MMSE may be expressed as:

$\begin{matrix}{{{\gamma_{{{sic} - {mmse}},m}(k)} = {\frac{q_{m\; m}^{l}(k)}{1 - {q_{m\; m}^{l}(k)}}{P_{m}(k)}}},{m = {1\mspace{14mu} \ldots \mspace{14mu} N_{S}}},} & {{Eq}.\mspace{14mu} (27)}\end{matrix}$

-   where q_(m,m) ^(l)(k) is the m-th diagonal element of Q_(sic)    ^(l)(k) for subband k, where Q_(sic) ^(l)(k) is derived as shown in    equation (21) but based on the reduced channel response matrix    H^(l)(k) instead of the original matrix H(k).

In general, the SNR progressively improves for data symbol streamsrecovered in later stages because the interference from data symbolstreams recovered in prior stages is canceled. This then allows higherrates to be used for data symbol streams recovered later.

Multi-User Steered Spatial Multiplexing Mode

The multi-user steered spatial multiplexing mode (or simply, the“multi-user steered mode”) supports data transmission from a singletransmitter to multiple receivers simultaneously based on “spatialsignatures” of the receivers. The spatial signature for a receiver isgiven by a channel response vector (for each subband) between the N_(T)transmit antennas and each receive antenna at the receiver. Thetransmitter may obtain the spatial signatures for the receivers asdescribed below. The transmitter may then (1) select a set of receiversfor simultaneous data transmission and (2) derive steering vectors forthe data symbol streams to be transmitted to the selected receivers suchthat transmit stream crosstalk is adequately suppressed at thereceivers.

The steering vectors for the multi-user steered mode may be derived invarious manners. Two exemplary schemes are described below. Forsimplicity, the following description is for one subband and assumesthat each receiver is equipped with one antenna.

In a channel inversion scheme, the transmitter obtains the steeringvectors for multiple receivers using channel inversion. The transmitterinitially selects N_(T) single-antenna receivers for simultaneoustransmission. The transmitter obtains a 1×N_(T) channel response rowvector h_(i)(k) for each selected receiver and forms an N_(T)×N_(T)channel response matrix H_(mu-s)(k) with the N_(T) row vectors for theN_(T) receivers. The transmitter then uses channel inversion to obtain amatrix F_(mu-s)(k) of N_(T) steering vectors for the N_(T) selectedreceivers, as follows:

F _(mu-s)(k)=H _(mu-s) ⁻¹(k).  Eq. (28)

The spatial processing at the transmitter for each subband for themulti-user steered mode may be expressed as:

x _(mu-s)(k)=F _(mu-s)(k)s(k).  Eq (29)

where x_(mu-s)(k) is the transmit symbol vector for the multi-usersteered mode.

The received symbols at the N_(T) selected receivers for each subbandmay be expressed as:

$\begin{matrix}\begin{matrix}{{{{\underset{\_}{r}}_{{mu} - s}(k)} = {{{{\underset{\_}{H}}_{{mu} - s}(k)}{{\underset{\_}{x}}_{{mu} - s}(k)}} + {\underset{\_}{n}(k)}}},} \\{{= {{{{\underset{\_}{H}}_{{mu} - s}(k)}{{\underset{\_}{F}}_{{mu} - s}(k)}{\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}}},} \\{{= {{\underset{\_}{s}(k)} + {\underset{\_}{i}(k)} + {\underset{\_}{n}(k)}}},}\end{matrix} & {{Eq}.\mspace{14mu} (30)}\end{matrix}$

-   where r_(mu-s)(k) is an (N_(T)×1) received symbol vector for subband    k at the N_(T) selected receivers, and i(k) represents the crosstalk    interference due to imperfect estimation of F_(mu-s)(k) at the    transmitter.    Each selected receiver would obtain only one entry of the vector    r_(mu-s)(k) for each receive antenna. If the spatial processing at    the transmitter is effective, then the power in i(k) is small, and    each recovered data symbol stream experiences little crosstalk from    the (N_(T)−1) other data symbol streams sent to the other receivers.

The transmitter can also transmit a steered pilot to each selectedreceiver, as described below. Each receiver would then process itssteered pilot to estimate the channel gain and phase and coherentlydemodulate the received symbols from its single antenna with the channelgain and phase estimates to obtain recovered data symbols.

The SNRs achieved for the multi-user steered mode are a function of theautocovariance of the channel response matrix H_(mu-s)(k). Higher SNRscan be achieved by selecting “compatible” user terminals. Different setsand/or combinations of user terminals may be evaluated, and theset/combination with the highest SNRs may be selected for datatransmission.

While the channel inversion scheme is appealing in its simplicity, ingeneral, it will provide poor performance, because preconditioning thedata symbol streams with the inverse channel response matrix in equation(29) forces the transmitter to put the majority of its power in theworst eigenmodes of the MIMO channel. Also, in some channels,particularly those with large correlations among the elements ofH_(mu-s)(k), the channel response matrix is less than full rank, andcalculating an inverse will not be possible.

In a precoding scheme, the transmitter precodes N_(T) data symbolstreams to be sent to the N_(T) selected receivers such that these datasymbol streams experience little crosstalk at the receivers. Thetransmitter can form the channel response matrix H_(mu)(k) for the N_(T)selected receivers. The transmitter then performs QR factorization onH_(mu)(k) such that H_(mu)(k)=F_(tri)(k)Q_(mu)(k), where F_(tri)(k) is alower left triangular matrix and Q_(mu)(k) is a unitary matrix.

The transmitter performs a precoding operation on the data symbol vectorto be transmitted, s(k)=[s₁(k) s₂ (k) . . . s_(N) _(T) (k)]^(T), toobtain a precoded symbol vector a(k)=[a₁(k) a₂(k) . . . a_(N) _(T)(k)]^(T), as follows:

$\begin{matrix}{{{a_{l}(k)} = {\frac{1}{f_{ll}(k)}\left( {{s_{l}(k)} - {\sum\limits_{i = 1}^{l - 1}{{f_{li}(k)}{a_{i}(k)}}}} \right)\; {{mod}\left( {M/2} \right)}}},{{{for}\mspace{14mu} l} = {1\mspace{14mu} \ldots \mspace{14mu} N_{T}}},} & {{Eq}.\mspace{14mu} (31)}\end{matrix}$

where

-   -   M is the number of levels, spaced at unit intervals, in the        in-phase or quadrature dimension of a square QAM signal        constellation; and    -   f_(li)(k) is the element of F_(tri)(k) in row i and column j.        The modulo (mod) operation adds a sufficient number of integer        multiples of M to the argument so that the result satisfies        a_(l)(k)ε[−M/2, M/2). After this precoding operation, the        transmit symbols are computed by processing the precoded symbol        vector a(k) with the unitary steering matrix Q_(mu)(k) to        generate the transmit symbol vector x_(mu-pc)(k)=Q_(mu)        ^(H)(k)a(k).

The receive symbol vector for the precoding scheme can be expressed as:

r _(mu-pc)(k)=H _(mu)(k)Q _(mu) ^(H)(k)a(k)+n(k)=F_(tri)(k)a(k)+n(k).  Eq. (32)

It can be shown that F_(tri)(k)a(k)mod(M/2)=s(k). Thus, the data symbolvector can be estimated as ŝ_(mu-pc)(k)=r_(mu-pc)(k)mod(M/2). Each ofthe N_(T) selected receivers only obtains one of the N_(T) elements ofr_(mu-pc)(k) and can estimate the data symbols sent to it by performingthe mod(M/2) operation on its received symbols.

The transmitter can also transmit multiple data symbol streams to amulti-antenna receiver in the multi-user steered mode. The channelresponse matrix H_(mu)(k) would then include one row vector for eachreceive antenna of the multi-antenna receiver.

The multi-user steered mode also supports data transmission frommultiple multi-antenna transmitters to a single receiver. Eachmulti-antenna transmitter performs spatial processing on its data symbolstream to steer the stream toward the receiver. Each transmitter alsotransmits a steered pilot to the receiver. To the receiver, eachtransmitter appears as a single transmission. The receiver performsspatial processing (e.g., CCMI, MMSE, and so on) to recover the steereddata symbol streams from all transmitters.

Multi-User Non-Steered Spatial Multiplexing Mode

The multi-user non-steered spatial multiplexing mode (or simply, the“multi-user non-steered mode”) supports simultaneous data transmissionby (1) a single transmitter to multiple receivers (e.g., for thedownlink) and (2) multiple transmitters to a single receiver (e.g., forthe uplink).

For non-steered transmission from a single transmitter to multiplereceivers, the transmitter transmits one data symbol stream from eachtransmit antenna for a recipient receiver. One or multiple data symbolstreams may be transmitted for each recipient receiver. Each recipientreceiver includes at least N_(T) receive antennas and can performspatial processing to isolate and recover its data symbol stream(s).Each receiver desiring data transmission estimates the SNR for each ofthe N_(T) transmit antennas and sends the N_(T) SNR estimates to thetransmitter. The transmitter selects a set of receivers for datatransmission based on the SNR estimates from all receivers desiring datatransmission (e.g., to maximize the overall throughput).

For non-steered transmission from multiple transmitters to a singlereceiver, the transmitters transmit data symbol streams from theirantennas (i.e., without spatial processing) such that these streamsarrive approximately time-aligned at the receiver. The receiver canestimate the channel response matrix for all of the transmitters as ifthey were one transmitter. The receiver can recover multiple data symbolstreams transmitted by these multiple transmitters using any of thetechniques described above for the single-user non-steered mode (e.g.,CCMI, MMSE, and SIC techniques).

Spatial Processing

Table 2 summarizes the spatial processing at the transmitter and thereceiver for the four spatial multiplexing modes described above. Forthe non-steered modes, receiver processing techniques other than CCMIand MMSE may also be used. The last column in Table 2 indicates whetheror not the SIC technique may be used at the receiver.

TABLE 2 Spatial Transmit Receive Multiplexing Mode F(k) M(k) Scaling SICSingle-User Steered V(k) U^(H)(k) Σ⁻¹(k) no Single-User I M_(ccmi)(k) —yes Non-Steered M_(mmse)(k) D_(mmse) ⁻¹(k) Multi-User Steered H_(mu−s)⁻¹(k) — — no (single transmitter to multiple receivers) Multi-User IM_(ccmi)(k) — yes Non-Steered M_(mmse)(k) D_(mmse) ⁻¹(k) (multipletransmitters to single receiver)For simplicity, the spatial processing for the multi-user steered modefrom multiple transmitters to a single receiver and the multi-usernon-steered mode from a single transmitter to multiple receivers are notshown in Table 2.

In the following description, a wideband spatial channel can correspondto (1) a wideband eigenmode, for a steered spatial multiplexing mode,(2) a transmit antenna, for a non-steered spatial multiplexing mode, or(3) a combination of one or more spatial channels of one or moresubbands. A wideband spatial channel can be used to transmit oneindependent data stream.

MIMO System

FIG. 1 shows a multiple-access MIMO system 100 with a number of accesspoints (APs) 110 providing communication for a number of user terminals(UTs) 120. For simplicity, only two access points 110 a and 110 b areshown in FIG. 1. An access point is generally a fixed station thatcommunicates with the user terminals and may also be referred to as abase station or some other terminology. A user terminal may be fixed ormobile and may also be referred to as a mobile station, a wirelessdevice, or some other terminology. A system controller 130 couples toand provides coordination and control for access points 110.

MIMO system 100 may be a time division duplex (TDD) system or afrequency division duplex (FDD) system. The downlink and uplink (1)share the same frequency band for a TDD system and (2) use differentfrequency bands for an FDD system. The following description assumesthat MIMO system 100 is a TDD system.

MIMO system 100 utilizes a set of transport channels to transmitdifferent types of data. The transport channels may be implemented invarious manners.

FIG. 2 shows an exemplary frame and channel structure 200 that may beused for MIMO system 100. Data transmission occurs in TDD frames. EachTDD frame spans a predetermined time duration (e.g., 2 msec) and ispartitioned into a downlink phase and an uplink phase. Each phase isfurther partitioned into multiple segments 210, 220, 230, 240, and 250for multiple transport channels.

In the downlink phase, a broadcast channel (BCH) carries a beacon pilot214, a MIMO pilot 216, and a BCH message 218. The beacon pilot is usedfor timing and frequency acquisition. The MIMO pilot is used for channelestimation. The BCH message carries system parameters for the userterminals. A forward control channel (FCCH) carries schedulinginformation for assignments of downlink and uplink resources and othersignaling for the user terminals. A forward channel (FCH) carries FCHprotocol data units (PDUs) on the downlink. An FCH PDU 232 a includes apilot 234 a and a data packet 236 a, and an FCH PDU 232 b includes onlya data packet 236 b. In the uplink phase, a reverse channel (RCH)carries RCH PDUs on the uplink. An RCH PDU 242 a includes only a datapacket 246 a, and an RCH PDU 242 b includes a pilot 244 b and a datapacket 246 b. A random access channel (RACH) is used by the userterminals to gain access to the system and to send short messages on theuplink. An RACH PDU 252 sent on the RACH includes a pilot 254 and amessage 256.

FIG. 3 shows a block diagram of an access point 110 x and two userterminals 120 x and 120 y in MIMO system 100. Access point 110 x is oneof the access points in FIG. 1 and is equipped with multiple (N_(ap))antennas 324 a through 324 ap. User terminal 120 x is equipped with asingle antenna 352 x, and user terminal 120 y is equipped with multiple(N_(ut)) antennas 352 a through 352 ut.

On the downlink, at access point 110 x, a TX data processor 310 receivestraffic data for one or more user terminals from a data source 308,control data from a controller 330, and possibly other data from ascheduler 334. The various types of data may be sent on differenttransport channels. TX data processor 310 processes (e.g., encodes,interleaves, and symbol maps) the different types of data based on oneor more coding and modulation schemes to obtain N_(S) streams of datasymbols. As used herein, a “data symbol” refers to a modulation symbolfor data, and a “pilot symbol” refers to a modulation symbol for pilot.A TX spatial processor 320 receives the N_(S) data symbol streams fromTX data processor 310, performs spatial processing on the data symbolswith matrices F_(ap)(k), for k=1 . . . N_(F), multiplexes in pilotsymbols, and provides N_(ap) streams of transmit symbols for the N_(ap)antennas. The matrices F_(ap)(k) are derived in accordance with thespatial multiplexing mode selected for use. The processing by TX dataprocessor 310 and TX spatial processor 320 is described below.

Each modulator (MOD) 322 receives and processes a respective transmitsymbol stream to obtain a stream of OFDM symbols, and further conditions(e.g., amplifies, filters, and frequency upconverts) the OFDM symbolstream to generate a downlink signal. N_(ap) modulators 322 a through322 ap provide N_(ap) downlink signals for transmission from N_(ap)antennas 324 a through 324 ap, respectively, to the user terminals.

At each user terminal 120, one or multiple antennas 352 receive theN_(ap) downlink signals, and each antenna provides a received signal toa respective demodulator (DEMOD) 354. Each demodulator 354 performsprocessing complementary to that performed by modulator 322 and providesa stream of received symbols. For single-antenna user terminal 120 x, anRX spatial processor 360 x performs coherent demodulation of thereceived symbol stream from a single demodulator 354 x and provides onestream of recovered data symbols. For multi-antenna user terminal 120 y,RX spatial processor 360 y performs spatial processing on N_(ut)received symbol streams from N_(ut) demodulators 354 with spatial filtermatrices M_(ut)(k), for k=1 . . . N_(F), and provides N_(ut) streams ofrecovered data symbols. In any case, each recovered data symbol stream{ŝ_(m)} is an estimate of a data symbol stream {s_(m)} transmitted byaccess point 110 x to that user terminal 120. An RX data processor 370receives and demultiplexes the recovered data symbols to the propertransport channels. The recovered data symbols for each transportchannel are then processed (e.g., demapped, deinterleaved, and decoded)to obtain decoded data for that transport channel. The decoded data foreach transport channel may include recovered traffic data, control data,and so on, which may be provided to a data sink 372 for storage and/or acontroller 380 for further processing.

At each user terminal 120, a channel estimator 378 estimates thedownlink channel response and provides channel estimates, which mayinclude channel gain estimates, SNR estimates, and so on. Controller 380receives the channel estimates, derives the vectors and/or coefficientsused for spatial processing on the transmit and receive paths, anddetermines a suitable rate for each data symbol stream on the downlink.For example, controller 380 y for multi-antenna user terminal 120 y mayderive the spatial filter matrices M_(ut)(k) for the downlink and thematrices F_(ut)(k) of steering vectors for the uplink based on downlinkchannel response matrices H_(dn)(k), for k=1 . . . N_(F). Controller 380may also receive the status of each packet/frame received on thedownlink and assemble feedback information for access point 110 x. Thefeedback information and uplink data are processed by a TX dataprocessor 390, spatially processed by a TX spatial processor 392 (ifpresent at user terminal 120), multiplexed with pilot symbols,conditioned by one or more modulators 354, and transmitted via one ormore antennas 352 to access point 110 x.

At access point 110 x, the transmitted uplink signals are received byantennas 324, demodulated by demodulators 322, and processed by an RXspatial processor 340 and an RX data processor 342 in a complementarymanner to that performed at user terminals 120. The recovered feedbackinformation is provided to controller 330 and scheduler 334. Scheduler334 may use the feedback information to perform a number of functionssuch as (1) scheduling a set of user terminals for data transmission onthe downlink and uplink and (2) assigning the available downlink anduplink resources to the scheduled terminals.

Controllers 330 and 380 control the operation of various processingunits at access point 110 x and user terminal 120, respectively. Forexample, controller 380 may determine the highest rates supported by thespatial channels on the downlink for user terminal 120. Controller 330may select the rate, payload size, and OFDM symbol size for each spatialchannel of each scheduled user terminal.

The processing at access point 110 x and user terminals 120 x and 120 yfor the uplink may be the same or different from the processing for thedownlink. For clarity, the processing for the downlink is described indetail below.

FIG. 4 shows a block diagram of an embodiment of TX data processor 310at access point 110 x. For this embodiment, TX data processor 310includes one set of encoder 412, channel interleaver 414, and symbolmapping unit 416 for each of the N_(S) data streams. For each datastream {d_(m)}, where m=1 . . . N_(S), an encoder 412 receives and codesthe data stream based on a coding scheme selected for that stream andprovides code bits. The coding scheme may include CRC, convolutional,Turbo, low density parity check (LDPC), block, and other coding, or acombination thereof. A channel interleaver 414 interleaves (i.e.,reorders) the code bits based on an interleaving scheme. A symbolmapping unit 416 maps the interleaved bits based on a modulation schemeselected for that stream and provides a stream of data symbols {s_(m)}.Unit 416 groups each set of B interleaved bits to form a B-bit binaryvalue, where B≧1, and further maps each B-bit binary value to a specificdata symbol based on the selected modulation scheme (e.g., QPSK, M-PSK,or M-QAM, where M=2^(B)). The coding and modulation for each data streamare performed in accordance with coding and modulation controls providedby controller 330.

FIG. 5 shows a block diagram of an embodiment of TX spatial processor320 and modulators 322 a through 322 ap at access point 110 x. For thisembodiment, TX spatial processor 320 includes N_(S) demultiplexers(Demux) 510 a through 510 s, N_(F) TX subband spatial processors 520 athrough 520 f, and N_(ap) multiplexers (Mux) 530 a through 530 ap. Eachdemultiplexer 510 receives a respective data symbol stream {s_(m).} fromTX spatial processor 320, demultiplexes the stream into N_(F) datasymbol substreams for the N_(F) subbands, and provides the N_(F)substreams to N_(F) spatial processors 520 a through 520 f. Each spatialprocessor 520 receives N_(S) data symbol substreams for its subband fromN_(S) demultiplexers 510 a through 510 s, performs transmitter spatialprocessing on these substreams, and provides N_(ap) transmit symbolsubstreams for the N_(ap) access point antennas. Each spatial processor520 multiplies a data vector s_(dn)(k) with a matrix F_(ap)(k) to obtaina transmit vector x_(dn)(k). The matrix F_(ap)(k) is equal to (1) amatrix V_(dn)(k) of right eigenvectors of H_(dn)(k) for the single-usersteered mode, (2) the matrix F_(mu)(k) for the multi-user steered mode,or (3) the identity matrix I for the single-user non-steered mode.

Each multiplexer 530 receives N_(F) transmit symbol substreams for itstransmit antenna from N_(F) spatial processors 520 a through 520 f,multiplexes these substreams and pilot symbols, and provides a transmitsymbol stream {x_(j)} for its transmit antenna. The pilot symbols may bemultiplexed in frequency (i.e., on some subbands), in time (i.e., insome symbol periods), and/or in code space (i.e., with an orthogonalcode). N_(ap) multiplexers 530 a through 530 ap provide N_(ap) transmitsymbol streams {x_(j)}, for j=1 . . . N_(ap), for N_(ap) antennas 324 athrough 324 ap.

For the embodiment shown in FIG. 5, each modulator 322 includes aninverse fast Fourier transform (IFFT) unit 542, a cyclic prefixgenerator 544, and a TX RF unit 546. IFFT unit 542 and cyclic prefixgenerator 544 form an OFDM modulator. Each modulator 322 receives arespective transmit symbol stream {x_(j)} from TX spatial processor 320and groups each set of N_(F) transmit symbols for the N_(F) subbands.IFFT unit 542 transforms each set of N_(F) transmit symbols to the timedomain using an NJ, point inverse fast Fourier transform and provides acorresponding transformed symbol that contains N_(F) chips. Cyclicprefix generator 544 repeats a portion of each transformed symbol toobtain a corresponding OFDM symbol that contains N_(F)+N_(cp) chips. Therepeated portion (i.e., the cyclic prefix) ensures that the OFDM symbolretains its orthogonal properties in the presence of multipath delayspread caused by frequency selective fading. TX RF unit 546 receives andconditions the OFDM symbol stream from generator 544 to generate adownlink modulated signal. N_(ap) downlink modulated signals aretransmitted from N_(ap) antennas 324 a through 324 ap, respectively.

FIG. 6 shows a block diagram of an embodiment of demodulators 354 athrough 354 ut and RX spatial processor 360 y for multi-antenna userterminal 120 y. At user terminal 120 y, N_(ut) antennas 352 a through352 ut receive the N_(ap) modulated signals transmitted by access point110 x and provide N_(ut) received signals to N_(ut) demodulators 354 athrough 354 ut, respectively. Each demodulator 354 includes an RX RFunit 612, a cyclic prefix removal unit 614, and a fast Fourier transform(FFT) unit 616. Units 614 and 616 form an OFDM demodulator. Within eachdemodulator 354, RX RF unit 612 receives, conditions, and digitizes arespective received signal and provides a stream of chips. Cyclic prefixremoval unit 614 removes the cyclic prefix in each received OFDM symbolto obtain a received transformed symbol. FFT unit 616 then transformseach received transformed symbol to the frequency domain with anN_(F)-point fast Fourier transform to obtain N_(F) received symbols forthe N_(F) subbands. FFT unit 616 provides a stream of received symbolsto RX spatial processor 360 y and received pilot symbols to channelestimator 378 y.

For the embodiment shown in FIG. 6, RX spatial processor 360 y includesN_(ut) demultiplexers 630 a through 630 ut for the N_(ut) antennas atuser terminal 120 y, N_(F) RX subband spatial processors 640 a through640 f and N_(F) scaling units 642 a through 642 f for the N_(F)subbands, and N_(S) multiplexers 650 a through 650 s for the N_(S) datastreams. RX spatial processor 360 y obtains N_(ut) received symbolstreams {r_(i)}, for i=1 . . . N, from demodulators 354 a through 354ut. Each demultiplexer 630 receives a respective received symbol stream{r_(i)}, demultiplexes the stream into N_(F) received symbol substreamsfor the N_(F) subbands, and provides the N_(F) substreams to N_(F)spatial processors 640 a through 640 f. Each spatial processor 640obtains N_(ut) received symbol substreams for its subband from N_(ut)demultiplexers 630 a through 630 ut, performs receiver spatialprocessing on these substreams, and provides N_(S) detected symbolsubstreams for its subband. Each spatial processor 640 multiplies areceived vector r_(dn)(k) with a matrix M_(ut)(k) to obtain a detectedsymbol vector {tilde over (s)}_(dn)(k). The matrix M_(ut)(k) is equal to(1) a matrix U_(dn) ^(H)(k) of left eigenvectors of H_(dn)(k) for thesingle-user steered mode or (2) the matrix M_(ccmi)(k), M_(mmse)(k), orsome other matrix for the single-user non-steered mode.

Each scaling unit 642 receives N_(S) detected symbol substreams for itssubband, scales these substreams, and provides N_(S) recovered datasymbol substreams for its subband. Each scaling unit 642 performs thesignal scaling of the detected symbol vector {tilde over (s)}_(dn)(k)with a diagonal matrix D_(ut) ⁻¹(k) and provides the recovered datasymbol vector ŝ_(dn)(k). Each multiplexer 650 receives and multiplexesN_(F) recovered data symbol substreams for its data stream from N_(F)scaling units 642 a through 642 f and provides a recovered data symbolstream. N_(S) multiplexers 650 a through 650 s provide N_(S) recovereddata symbol streams {ŝ_(m)}, for m=1 . . . N_(S).

FIG. 7 shows a block diagram of an embodiment of RX data processor 370 yat user terminal 120 y. RX data processor 370 y includes one set ofsymbol demapping unit 712, channel deinterleaver 714, and decoder 716for each of the N_(S) data streams. For each recovered data symbolstream {ŝ_(m)}, where m=1 . . . N_(S), a symbol demapping unit 712demodulates the recovered data symbols in accordance with the modulationscheme used for that stream and provides demodulated data. A channeldeinterleaver 714 deinterleaves the demodulated data in a mannercomplementary to the interleaving performed on that stream by accesspoint I 110 x. A decoder 716 then decodes the deinterleaved data in amanner complementary to the encoding performed by access point 110 x onthat stream. For example, a Turbo decoder or a Viterbi decoder may beused for decoder 716 if Turbo or convolutional coding, respectively, isperformed at access point 110 x. Decoder 716 provides a decoded packetfor each received data packet. Decoder 716 further checks each decodedpacket to determine whether the packet is decoded correctly or in errorand provides the status of the decoded packet. The demodulation anddecoding for each recovered data symbol stream are performed inaccordance with demodulation and decoding controls provided bycontroller 380 y.

FIG. 8 shows a block diagram of an RX spatial processor 360 z and an RXdata processor 370 z, which implement the SIC technique. RX spatialprocessor 360 z and RX data processor 370 z implement N_(S) successive(i.e., cascaded) receiver processing stages for N_(S) data symbolstreams. Each of stages 1 to N_(S)−1 includes a spatial processor 810,an interference canceller 820, an RX data stream processor 830, and a TXdata stream processor 840. The last stage includes only a spatialprocessor 810 s and an RX data stream processor 830 s. Each RX datastream processor 830 includes a symbol demapping unit 712, a channeldeinterleaver 714, and a decoder 716, as shown in FIG. 7. Each TX datastream processor 840 includes an encoder 412, a channel interleaver 414,and a symbol mapping unit 416, as shown in FIG. 4.

For stage 1, spatial processor 810 a performs receiver spatialprocessing on the N_(ut) received symbol streams and provides onerecovered data symbol stream {ŝ_(j) ₁ }, where the subscript j₁ denotesthe access point antenna used to transmit the data symbol stream {s_(j)₁ }. RX data stream processor 830 a demodulates, deinterleaves, anddecodes the recovered data symbol stream {ŝ_(j) ₁ } and provides acorresponding decoded data stream {{circumflex over (d)}_(j) ₁ }. TXdata stream processor 840 a encodes, interleaves, and modulates thedecoded data stream {{circumflex over (d)}_(j) ₁ } in the same mannerperformed by access point 110 x for that stream and provides aremodulated symbol stream {{hacek over (s)}_(j) ₁ }. Interferencecanceller 820 a performs spatial processing on the remodulated symbolstream {{hacek over (s)}_(j) ₁ } in the same manner (if any) performedby access point 110 x and further processes the result with the channelresponse matrix H_(dn)(k) to obtain N_(ut) interference components dueto the data symbol stream {s_(j) ₁ }. The N_(ut) interference componentsare subtracted from the Nat received symbol streams to obtain N_(ut)modified symbol streams, which are provided to stage 2.

Each of stages 2 through N_(S)−1 performs the same processing as stage1, albeit on the N_(ut) modified symbol streams from the preceding stageinstead of the N_(ut) received symbol streams. The last stage performsspatial processing and decoding on the N_(ut) modified symbol streamsfrom stage N_(S)−1 and does not perform interference estimation andcancellation.

Spatial processors 810 a through 810 s may each implement the CCMI,MMSE, or some other receiver processing technique. Each spatialprocessor 810 multiplies an input (received or modified) symbol vectorr_(dn) ^(l)(k) with a matrix M_(ut) ^(l)(k) to obtain a detected symbolvector {tilde over (s)}_(dn) ^(l)(k), selects and scales one of thedetected symbol streams, and provides the scaled symbol stream as therecovered data symbol stream for that stage. The matrix M_(ut) ^(l)(k)is derived based on a reduced channel response matrix H_(dn) ^(l)(k) forthe stage.

The processing units at access point 110 x and user terminal 120 y forthe uplink may be implemented as described above for the downlink. TXdata processor 390 y and TX spatial processor 392 y may be implementedwith TX data processor 310 in FIG. 4 and TX spatial processor 320 inFIG. 5, respectively. RX spatial processor 340 may be implemented withRX spatial processor 360 y or 360 z, and RX data processor 342 may beimplemented with data processor 370 y or 370 z.

For single-antenna user terminal 120 x, RX spatial processor 360 xperforms coherent demodulation of one received symbol stream withchannel estimates to obtain one recovered data symbol stream.

Channel Estimation

The channel response of the downlink and uplink may be estimated invarious manners such as with a MIMO pilot or a steered pilot. For a TDDMIMO system, certain techniques may be used to simplify the channelestimation.

For the downlink, access point 110 x can transmit a MIMO pilot to userterminals 120. The MIMO pilot comprises N_(ap) pilot transmissions fromN_(ap) access point antennas, with the pilot transmission from eachantenna being “covered” with a different orthogonal sequence (e.g., aWalsh sequence). Covering is a process whereby a given modulation symbol(or a set of L modulation symbols with the same value) to be transmittedis multiplied by all L chips of an L-chip orthogonal sequence to obtainL covered symbols, which are then transmitted. The covering achievesorthogonality among the N_(ap) pilot transmissions sent from the N_(ap)access point antennas and allows the user terminals to distinguish thepilot transmission from each antenna.

At each user terminal 120, channel estimator 378 “decovers” the receivedpilot symbols for each user terminal antenna i with the same N_(ap)orthogonal sequences used by access point 110 x for the N_(ap) antennasto obtain estimates of the complex channel gain between user terminalantenna i and each of the N_(ap) access point antennas. Decovering iscomplementary to covering and is a process whereby received (pilot)symbols are multiplied by the L chips of the L-chip orthogonal sequenceto obtain L decovered symbols, which are then accumulated to obtain anestimate of the transmitted (pilot) symbol. Channel estimator 378performs the same pilot processing for each subband used for pilottransmission. If pilot symbols are transmitted on only a subset of theN_(F) subbands, then channel estimator 378 can perform interpolation onthe channel response estimates for subbands with pilot transmission toobtain channel response estimates for subbands without pilottransmission. For single-antenna user terminal 120 x, channel estimator378 x provides estimated downlink channel response vectors ĥ_(dn)(k),for k=1 . . . N_(F), for the single antenna 352. For multi-antenna userterminal 120 y, channel estimator 378 y performs the same pilotprocessing for all N_(ut) antennas 352 a through 352 ut and providesestimated downlink channel response matrices Ĥ_(dn)(k), for k=1 . . .N_(F). Each user terminal 120 can also estimate the noise variance forthe downlink based on the received pilot symbols and provides thedownlink noise estimate, {circumflex over (σ)}_(dn) ².

For the uplink, multi-antenna user terminal 120 y can transmit a MIMOpilot that can be used by access point 110 x to estimate the uplinkchannel response Ĥ_(up)(k) for user terminal 120 y. Single-antenna userterminal 120 x can transmit a pilot from its single antenna. Multiplesingle-antenna user terminals 120 can transmit orthogonal pilotssimultaneously on the uplink, where orthogonality may be achieved intime and/or frequency. Time orthogonality can be obtained by having eachuser terminal cover its uplink pilot with a different orthogonalsequence assigned to the user terminal. Frequency orthogonality can beobtained by having each user terminal transmit its uplink pilot on adifferent set of subbands. The simultaneous uplink pilot transmissionsfrom multiple user terminals should be approximately time-aligned ataccess point 120 x (e.g., time-aligned to within the cyclic prefix).

For a TDD MIMO system, a high degree of correlation normally existsbetween the channel responses for the downlink and uplink since theselinks share the same frequency band. However, the responses of thetransmit/receive chains at the access point are typically not the sameas the responses of the transmit/receive chains at the user terminal. Ifthe differences are determined and accounted for via calibration, thenthe overall downlink and uplink channel responses may be assumed to bereciprocal (i.e., transpose) of each other.

FIG. 9 shows the transmit/receive chains at access point 110 x and userterminal 120 y. At access point 110 x, the transmit path is modeled byan N_(ap)×N_(ap) matrix T_(ap)(k) and the receive path is modeled by anN_(ap)×N_(ap) matrix R_(ap)(k). At user terminal 120 y, the receive pathis modeled by an N_(ut)×N_(ut) matrix R_(ut)(k) and the transmit path ismodeled by an N_(ut)×N_(ut) matrix T_(ut)(k). The received symbolvectors for the downlink and uplink for each subband may be expressedas:

r _(dn)(k)=R _(ut)(k)H(k)T _(ap)(k)x _(dn)(k), and

r _(up)(k)=R _(ap)(k)H ^(T)(k)T _(ut)(k)x _(up)(k),  Eq. (33)

-   where “^(T)” denotes the transpose. Equation (34) assumes that the    downlink and uplink are transpose of one another. The “effective”    downlink and uplink channel responses, H_(edn)(k) and H_(eup)(k),    for each subband include the responses of the transmit and receive    chains and may be expressed as:

H _(edn)(k)=R _(ut)(k)H(k)T _(ap)(k) and H _(eup)(k)=R _(ap)(k)H^(T)(k)T _(ut)(k).  Eq. (34)

The effective downlink and uplink channel responses are not reciprocalof one other (i.e., H_(edn)(k)≠H_(eup) ^(T)(k)) if the responses of thedownlink and uplink transmit/receive chains are not equal to each other.

Access point 110 x and user terminal 120 y can perform calibration toobtain correction matrices K_(ap)(k) and K_(ut)(k) for each subband,which may be expressed as:

K _(ap)(k)=T _(ap) ⁻¹(k)R _(ap)(k) and K _(ut)(k)=T _(ut) ⁻¹(k)R_(ut)(k).  Eq. (35)

The correction matrices may be obtained by transmitting MIMO pilots onboth the downlink and uplink and deriving the correction matrices usingMMSE criterion or some other techniques. The correction matricesK_(ap)(k) and K_(ut)(k) are applied at access point 110 x and userterminal 120 y, respectively, as shown in FIG. 9. The “calibrated”downlink and uplink channel responses, H_(cdn)(k) and H_(cup)(k), arethen reciprocal of one another and may be expressed as:

H _(cup)(k)=H _(up)(k)K _(ut)(k)=(H _(dn)(k)K _(ap)(k))^(T) =H _(cdn)^(T)(k).  Eq. (36)

The singular value decomposition of the calibrated uplink and downlinkchannel response matrices, H_(cup)(k) and H_(cdn)(k), for each subbandmay be expressed as:

H _(cup)(k)=U _(ap)(k)Σ(k)V _(ut) ^(H)(k), and

H _(cdn)(k)=V* _(ut)(k)Σ(k)U _(ap) ^(H)(k).  Eq. (37)

As shown in equation set (38), the matrices V*_(ut)(k) and U*_(ap)(k) ofleft and right eigenvectors of H_(cdn)(k) are the complex conjugate ofthe matrices V_(ut)(k) and U_(ap)(k) of right and left eigenvectors ofH_(cup)(k). The matrix U_(ap)(k) can be used by access point 110 x forboth transmit and receive spatial processing. The matrix V_(ut)(k) canbe used by user terminal 120 y for both transmit and receive spatialprocessing.

Because of the reciprocal nature of the MIMO channel for the TDD MIMOsystem, and after calibration has been performed to account for thedifferences in the transmit/receive chains, the singular valuedecomposition only needs to be performed by either user terminal 120 yor access point 110 x. If performed by user terminal 120 y, then thematrices V_(ut)(k), for k=1 . . . N_(F), are used for spatial processingat the user terminal and the matrix U_(ap)(k), for k=1 . . . N_(F), maybe provided to the access point in either a direct form (e.g., bysending entries of the matrices U_(ap)(k)) or an indirect form (e.g.,via a steered pilot). In actuality, user terminal 120 y can only obtainĤ_(cdn)(k), which is an estimate of H_(cdn)(k), and can only derive{circumflex over (V)}_(ut)(k), {circumflex over (Σ)}(k) and Û_(ap)(k),which are estimates of V_(ut)(k), Σ(k) and U_(ap)(k), respectively. Forsimplicity, the description herein assumes channel estimation withouterrors.

An uplink steered pilot sent by user terminal 120 y may be expressed as:

x _(up,m)(k)=K _(ut)(k)v _(ut,m)(k)p(k),  Eq. (38)

-   where v_(up,m)(k) is the m-th column of V_(ut)(k) and p(k) is the    pilot symbol. The received uplink steered pilot at access point 110    x may be expressed as:

r _(up,m)(k)=u _(ap,m)(k)σ_(m) p(k)+n _(up)(k)  Eq. (39)

Equation (40) indicates that access point 110 x can obtain the matrixU_(ap)(k), one vector at a time, based on the uplink steered pilot fromuser terminal 120 y.

A complementary process may also be performed whereby user terminal 120y transmits a MIMO pilot on the uplink, and access point 110 x performssingular value decomposition and transmits a steered pilot on thedownlink. Channel estimation for the downlink and uplink may also beperformed in other manners.

At each user terminal 120, channel estimator 378 can estimate thedownlink channel response (e.g., based on a MIMO pilot or a steeredpilot sent by access point 110 x) and provide downlink channel estimatesto controller 380. For single-antenna user terminal 120 x, controller380 x can derive the complex channel gains used for coherentdemodulation. For multi-antenna user terminal 120 y, controller 380 ycan derive the matrix M_(ut)(k) used for receive spatial processing andthe matrix F_(ut)(k) used for transmit spatial processing based on thedownlink channel estimates. At access point 110 x, channel estimator 328can estimate the uplink channel response (e.g., based on a steered pilotor a MIMO pilot sent by user terminal 120) and provide uplink channelestimates to controller 380. Controller 380 can derive the matrixF_(ap)(k) used for transmit spatial processing and the matrix M_(ap)(k)used for receive spatial processing based on the uplink channelestimates.

FIG. 9 shows the spatial processing at access point 110 x and userterminal 120 y for the downlink and uplink for one subband k. For thedownlink, within TX spatial processor 320 at access point 110 x, thedata vector s_(dn)(k) is first multiplied with the matrix F_(ap)(k) by aunit 910 and further multiplied with the correction matrix K_(ap)(k) bya unit 912 to obtain the transmit vector x_(dn)(k). The vector x_(dn)(k)is processed by a transmit chain 914 within modulators 322 andtransmitted over the MIMO channel to user terminal 120 y. Units 910 and912 perform the transmit spatial processing for the downlink and may beimplemented within TX subband spatial processor 520 in FIG. 5.

At user terminal 120 y, the downlink signals are processed by a receivechain 954 within demodulators 354 to obtain the receive vectorr_(dn)(k). Within RX spatial processor 360 y, the receive vectorr_(dn)(k) is first multiplied with the matrix M_(ut)(k) by a unit 956and further scaled with the inverse diagonal matrix D_(ut) ⁻¹(k) by aunit 958 to obtain the vector ŝ_(dn)(k), which is an estimate of thedata vector s_(dn)(k). Units 956 and 958 perform the receive spatialprocessing for the downlink and may be implemented within RX subbandspatial processor 640 in FIG. 6.

For the uplink, within TX spatial processor 392 y at user terminal 120y, the data vector s_(up)(k) is first multiplied with the matrixF_(ut)(k) by a unit 960 and further multiplied with the correctionmatrix K_(ut)(k) by a unit 962 to obtain the transmit vector x_(up)(k).The vector x_(up)(k) is processed by a transmit chain 964 withinmodulators 354 and transmitted over the MIMO channel to access point 110x. Units 960 and 962 perform the transmit spatial processing for theuplink.

At access point 110 x, the uplink signals are processed by a receivechain 924 within demodulators 322 to obtain the receive vectorr_(up)(k). Within RX spatial processor 340, the receive vector r_(up)(k)is first multiplied with the matrix M_(ap)(k) by a unit 926 and furtherscaled with the inverse diagonal matrix D_(ap) ⁻¹(k) by a unit 928 toobtain the vector ŝ_(up)(k), which is an estimate of the data vectors_(up)(k). Units 926 and 928 perform the receive spatial processing forthe uplink.

Spatial Processing for TDD MIMO System

Table 3 summarizes exemplary pilot transmission and spatial processingperformed by the access point and the user terminals for datatransmission on the downlink and uplink for various spatial multiplexingmodes in the TDD MIMO system. For the single-user steered mode, theaccess point transmits a MIMO pilot to allow the user terminal toestimate the downlink channel response. The user terminal transmits asteered pilot to allow the access point to estimate the uplink channelresponse. The access point performs transmit and receive spatialprocessing with U_(ap)(k). The user terminal performs transmit andreceive spatial processing with V_(ut)(k).

For the single-user non-steered mode, for downlink data transmission,the access point transmits a MIMO pilot from all antennas and a datasymbol stream from each antenna. The user terminal estimates thedownlink channel response with the MIMO pilot and performs receiverspatial processing using the downlink channel estimates. Thecomplementary processing occurs for uplink data transmission.

TABLE 3 Spatial Multiplexing Downlink Data Uplink Data Mode TransmissionTransmission Single-User AP transmits MIMO pilot AP transmits MIMO pilotSteered UT transmits steered pilot UT transmits steered pilot APtransmits data with U_(ap)(k) UT transmits data with V_(ut)(k) UTreceives data with V_(ut)(k) AP receives data with U_(ap)(k) Single-UserAP transmits MIMO pilot UT transmits MIMO pilot Non-Steered AP transmitsdata from each antenna UT transmits data from each antenna UT uses CCMI,MMSE, etc. AP uses CCMI, MMSE, etc. Multi-User UTs transmit orthogonalpilot AP transmits MIMO pilot Steered AP transmits steered data UTstransmit steered pilot AP transmits steered pilot UTs transmit steereddata UTs receive with steered pilot AP uses CCMI, MMSE, etc. Multi-UserAP transmits MIMO pilot UTs transmit orthogonal pilot Non-Steered UTssend rate for each AP antenna AP selects compatible UTs AP transmitsdata from each antenna UTs transmits data from each UTs use CCMI, MMSE,etc. antenna AP uses CCMI, MMSE, etc.

For the multi-user steered mode, for downlink data transmission tosingle-antenna and/or multi-antenna user terminals, the user terminalstransmit orthogonal pilots on the uplink to allow the access point toestimate the downlink channel response. A single-antenna user terminaltransmits an unsteered pilot, and a multi-antenna user terminaltransmits a steered pilot. The access point derives downlink steeringvectors based on the orthogonal uplink pilots, and uses the steeringvectors to transmit steered pilots and steered data symbol streams tothe selected user terminals. Each user terminal uses the steered pilotto receive the steered data symbol stream sent to the user terminal. Foruplink data transmission from multi-antenna user terminals, the accesspoint transmits a MIMO pilot. Each multi-antenna user terminal transmitsa steered pilot and a steered data symbol stream on the uplink. Theaccess point performs receiver spatial processing (e.g., CCMI, MMSE, andso on) to recover the data symbol streams.

For the multi-user non-steered mode, for downlink data transmission tomulti-antenna user terminals, the access point transmits a MIMO pilot onthe downlink. Each user terminal determines and sends back the rate itcan receive from each access point antenna. The access point selects aset of user terminals and transmits data symbol streams for the selecteduser terminals from the access point antennas. Each multi-antenna userterminal performs receiver spatial processing (e.g., CCMI, MMSE, and soon) to recover its data symbol stream. For uplink data transmission fromsingle-antenna and/or multi-antenna user terminals, the user terminalstransmit orthogonal (unsteered) pilots on the uplink. The access pointestimates the uplink channel response based on the uplink pilots andselects a set of compatible user terminals. Each selected user terminaltransmits a data symbol stream from a user terminal antenna. The accesspoint performs receiver spatial processing (e.g., CCMI, MMSE, and so on)to recover the data symbol streams.

Rate Selection

Each data stream for the downlink and uplink is transmitted on awideband spatial channel m using one of the spatial multiplexing modes.Each data stream is also transmitted at a rate that is selected suchthat the target level of performance (e.g., 1 percent packet error rate(PER)) can be achieved for that stream. The rate for each data streamcan be determined based on the SNR achieved at the receiver for thatstream (i.e., the received SNR), where the SNR is dependent on thespatial processing performed at the transmitter and receiver, asdescribed above.

In an exemplary rate selection scheme, the determine the rate forwideband spatial channel m, an SNR estimate, γ_(m)(k), (e.g., in unitsof dB) for each subband k of the wideband spatial channel is firstobtained, as described above. An average SNR, γ_(avg), is then computedfor wideband spatial channel m, as follows:

$\begin{matrix}{\gamma_{{avg},m} = {\frac{1}{N_{F}}{\sum\limits_{k = 1}^{N_{F}}{{\gamma_{m}(k)}.}}}} & {{Eq}.\mspace{14mu} (40)}\end{matrix}$

The variance of the SNR estimates, σ_(γm) ², is also computed asfollows:

$\begin{matrix}{\sigma_{\gamma_{m}}^{2} = {\frac{1}{N_{F} - 1}{\sum\limits_{k = 1}^{N_{F}}{\left( {{\gamma_{m}(k)} - \gamma_{{avg},m}} \right)^{2}.}}}} & {{Eq}.\mspace{14mu} (41)}\end{matrix}$

An SNR back-off factor, γ_(bo,m), is determined based on a functionF(γ_(avg,m,)σ_(γm) ²) of the average SNR and the SNR variance. Forexample, the function F(γ_(avg,m),σ_(γm) ²)=K_(b)·σ_(γm) ² may be used,where K_(b) is a scaling factor that may be selected based on one ormore characteristics of the MIMO system such as the interleaving, packetsize, and/or coding scheme used for the data stream. The SNR back-offfactor accounts for variation in SNRs across the wideband spatialchannel. An operating SNR, γ_(op,m), for wideband spatial channel m isnext computed, as follows:

γ_(op,m)=γ_(avg,m)−γ_(bo,m).  Eq. (42)

The rate for the data stream is then determined based on the operatingSNR. For example, a look-up table (LUT) may store a set of ratessupported by the MIMO system and their required SNRs. The required SNRfor each rate may be determined by computer simulation, empiricalmeasurement, and so on, and based on an assumption of an AWGN channel.The highest rate in the look-up table with a required SNR that is equalto or lower than the operating SNR is selected as the rate for the datastream sent on wideband spatial channel m.

Various other rate selection schemes may also be used.

Closed-Loop Rate Control

Closed-loop rate control may be used for each of the data streamstransmitted on multiple wideband spatial channels. Closed-loop ratecontrol may be achieved with one or multiple loops.

FIG. 10 shows a block diagram of an embodiment of a closed-loop ratecontrol mechanism 1000, which comprises an inner loop 1010 that operatesin conjunction with an outer loop 1020. Inner loop 1010 estimates thechannel conditions and determines the rate supported by each widebandspatial channel. Outer loop 1020 estimates the quality of the datatransmission received on each wideband spatial channel and adjusts theoperation of the inner loop accordingly. For simplicity, the operationof loops 1010 and 1020 for one downlink wideband spatial channel m isshown in FIG. 10 and described below.

For inner loop 1010, channel estimator 378 at user terminal 120estimates wideband spatial channel m and provides channel estimates(e.g., channel gain estimates and noise variance estimate). A rateselector 1030 within controller 380 determines the rate supported bywideband spatial channel m based on (1) the channel estimates fromchannel estimator 378, (2) an SNR back-off factor and/or a rateadjustment for wideband spatial channel m from a quality estimator 1032,and (3) a look-up table (LUT) 1036 of rates supported by the MIMO systemand their required SNRs. The supported rate for wideband spatial channelm is sent by controller 380 to access point 110. At access point 110,controller 330 receives the supported rate for wideband spatial channelm and determines the data rate, coding, and modulation controls for thedata stream to be sent on this spatial channel. The data stream is thenprocessed in accordance with these controls by TX data processor 310,spatially processed and multiplexed with pilot symbols by TX spatialprocessor 320, conditioned by modulators 322, and transmitted to userterminal 120.

Outer loop 1020 estimates the quality of the decoded data steam receivedon wideband spatial channel m and adjusts the operation of inner loop1010. The received symbols for wideband spatial channel m are spatiallyprocessed by RX spatial processor 360 and further processed by RX dataprocessor 370. RX data processor 370 provides the status of each packetreceived on wideband spatial channel m and/or decoder metrics to qualityestimator 1032. Outer loop 1020 can provide different types ofinformation (e.g., SNR back-off factor, a rate adjustment, and so on)used to control the operation of inner loop 1010.

Closed-loop rate control described above may thus be performedindependently for each downlink and uplink wideband spatial channel,which can correspond to (1) a wideband eigenmode, for the single-usersteered mode, or (2) a transmit antenna, for the single-user andmulti-user non-steered modes.

Scheduling User Terminals

FIG. 11 shows a block diagram of an embodiment of controller 330 andscheduler 334 for scheduling user terminals for data transmission on thedownlink and uplink. Within controller 330, a request processor 1110receives access requests transmitted by user terminal 120 on the RACHand possibly access requests from other sources. These access requestsare for data transmission on the downlink and/or uplink. Requestprocessor 1110 processes the received access requests and provides theidentities (IDs) and the status of all requesting user terminals. Thestatus for a user terminal may indicate the number of antennas availableat the terminal, whether the terminal is calibrated, and so on.

A rate selector 1120 receives channel estimates from channel estimator328 and determines the rates supported by the downlink and/or uplinkwideband spatial channels for the requesting user terminals, asdescribed above. For the downlink, each user terminal 120 can determinethe rate supported by each of its wideband spatial channels, asdescribed above. The supported rate is the maximum rate that may be usedfor data transmission on the wideband spatial channel to achieve thetarget level of performance. Each user terminal 120 can send thesupported rates for all of its downlink wideband spatial channels toaccess point 110, e.g., via the RACH. Alternatively, access point 110can determine the supported rates for the downlink wideband spatialchannels if (1) the downlink and uplink are reciprocal and (2) accesspoint 110 is provided with the noise variance or noise floor at userterminal 120. For the uplink, access point 110 can determine thesupported rate for each wideband spatial channel for each requestinguser terminal 120.

A user selector 1140 selects different sets of one or more userterminals, from among all of the requesting user terminals, for possibledata transmission on the downlink and/or uplink. The user terminals maybe selected based on various criteria such as system requirements, userterminal capabilities and supported rates, user priority, the amount ofdata to send, and so on. For the multi-user spatial multiplexing modes,the user terminals for each set may also be selected based on theirchannel response vectors.

A mode selector 1130 selects the particular spatial multiplexing mode touse for each set of user terminals based on the operating state andcapabilities of the user terminals in the set and possibly otherfactors. For example, the single-user steered mode may be used for a“calibrated” multi-antenna user terminal that has performed calibrationso that the channel response for one link (e.g., downlink) can beestimated based on a (e.g., steered) pilot received via the other link(e.g., uplink). The single-user non-steered mode may be used for an“uncalibrated” multi-antenna user terminal that has not performedcalibration or cannot support the single-user steered mode for anyreason. The multi-user steered mode may be used for downlinktransmission to multiple user terminals, each of which is equipped withone or more antennas. The multi-user non-steered mode may be used foruplink transmission by multiple user terminals.

Scheduler 334 receives the sets of user terminals from user selector1140, the selected spatial multiplexing mode for each user terminal setfrom mode selector 1130, and the selected rates for each user terminalset from rate selector 1120. Scheduler 334 schedules the user terminalsfor data transmission on the downlink and/or uplink. Scheduler 334selects one or more sets of user terminals for data transmission on thedownlink and one or more sets of user terminals for data transmission onthe uplink for each TDD frame. Each set includes one or more userterminals and is scheduled for data transmission concurrently in adesignated transmission interval within the TDD frame.

Scheduler 334 forms an information element (IE) for each user terminalscheduled for data transmission on the downlink and/or uplink. Eachinformation element includes (1) the spatial multiplexing mode to usefor data transmission, (2) the rate to use for the data stream sent oneach wideband spatial channel, (3) the start and the duration of thedata transmission, and (4) possibly other information (e.g., the type ofpilot being transmitted along with the data transmission). Scheduler 334sends the information elements for all scheduled user terminals via theFCCH. Each user terminal processes the FCCH to recover its informationelement, and thereafter receives a downlink transmission and/or sends anuplink transmission in accordance with the received schedulinginformation.

FIG. 11 shows an embodiment of the scheduling of user terminals for datatransmission when multiple spatial multiplexing modes are supported. Thescheduling may be performed in other manners, and this is within thescope of the invention.

FIG. 12 shows a flow diagram of a process 1200 for scheduling userterminals for data transmission in MIMO system 100. A set of least oneuser terminal is selected for data transmission on the downlink and/oruplink (block 1212). A spatial multiplexing mode is selected for theuser terminal set from among multiple spatial multiplexing modessupported by the system (block 1214). Multiple rates are also selectedfor multiple data streams to be transmitted via multiple spatialchannels for the user terminal set (block 1216). The user terminal setis scheduled for data transmission on the downlink and/or uplink withthe selected rates and the selected spatial multiplexing mode (block1218).

FIG. 13 shows a flow diagram of a process 1300 for transmitting data onthe downlink in MIMO system 100. Process 1300 may be performed by accesspoint 110 x. A first plurality of data streams are coded and modulatedin accordance with a first plurality of rates to obtain a firstplurality of data symbol streams (block 1312). For the single-usersteered mode, the first plurality of data symbol streams are spatiallyprocessed with a first plurality of steering vectors to obtain a firstplurality of transmit symbol streams for transmission from multipleantennas to a first user terminal in a first transmission interval(block 1314). The first plurality of steering vectors are derived suchthat the first plurality of data streams are transmitted on orthogonalspatial channels to the first user terminal. A second plurality of datastreams are coded and modulated in accordance with a second plurality ofrates to obtain a second plurality of data symbol streams (block 1316).For the single-user non-steered mode, the second plurality of datasymbol streams are provided as a second plurality of transmit symbolstreams for transmission from the multiple antennas to a second userterminal in a second transmission interval (block 1318). A thirdplurality of data streams are coded and modulated to obtain a thirdplurality of data symbol streams (block 1320). For the multi-usersteered mode, the third plurality of data symbol streams are spatiallyprocessed with a second plurality of steering vectors to obtain a thirdplurality of transmit symbol streams for transmission from the multipleantennas to multiple user terminals in a third transmission interval(block 1322). The second plurality of steering vectors are derived suchthat the third plurality of data symbol streams are received withsuppressed crosstalk at the multiple user terminals.

FIG. 14 shows a flow diagram of a process 1400 for receiving data on theuplink in MIMO system 100. Process 1400 may also be performed by accesspoint 110 x. Receiver spatial processing is performed on a firstplurality of received symbol streams in accordance with a first spatialmultiplexing mode (e.g., the single-user steered mode) to obtain a firstplurality of recovered data symbol streams (block 1412). The firstplurality of recovered data symbol streams are demodulated and decodedin accordance with a first plurality of rates to obtain a firstplurality of decoded data streams (block 1414). Receiver spatialprocessing is performed on a second plurality of received symbol streamsin accordance with a second spatial multiplexing mode (e.g., anon-steered mode) to obtain a second plurality of recovered data symbolstreams (block 1416). The second plurality of recovered data symbolstreams are demodulated and decoded in accordance with a secondplurality of rates to obtain a second plurality of decoded data streams,which are estimates of data streams transmitted by one or multiple userterminals (block 1418).

Each user terminal performs corresponding processes to transmit data onone or multiple uplink wideband spatial channels and to receive data onone or multiple downlink wideband spatial channels.

Data transmission with multiple spatial multiplexing modes, as describedherein, may be implemented by various means. For example, the processingmay be implemented in hardware, software, or a combination thereof. Fora hardware implementation, the processing units used to perform dataprocessing, spatial processing, and scheduling at the access point maybe implemented within one or more application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), fieldprogrammable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof. Theprocessing units at a user terminal may also be implemented on one ormore ASICs, DSPs, and so on.

For a software implementation, the processing at the access point anduser terminal for data transmission with multiple spatial multiplexingmodes may be implemented with modules (e.g., procedures, functions, andso on) that perform the functions described herein. The software codesmay be stored in a memory unit (e.g., memory unit 332 or 382 in FIG. 3)and executed by a processor (e.g., controller 330 or 380). The memoryunit may be implemented within the processor or external to theprocessor.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of transmitting data in a wireless multiple-accessmultiple-input multiple-output (MIMO) communication system, comprising:selecting at least one user terminal for data transmission; selecting aspatial multiplexing mode, from among a plurality of spatialmultiplexing modes supported by the system, to use for the at least oneuser terminal; selecting a plurality of rates for a plurality of datastreams to be transmitted via a plurality of spatial channels of a MIMOchannel for the at least one user terminal; and scheduling the at leastone user terminal for data transmission with the plurality of selectedrates and the selected spatial multiplexing mode. different spatialmultiplexing modes
 2. The method of claim 1, wherein one user terminalis selected for data transmission and the selected spatial multiplexingmode is a steered spatial multiplexing mode.
 3. The method of claim 2,further comprising: spatially processing the plurality of data streamswith a plurality of steering vectors to transmit the plurality of datastreams on orthogonal spatial channels to the one user terminal.
 4. Themethod of claim 1, wherein one user terminal is selected for datatransmission and the selected spatial multiplexing mode is a non-steeredspatial multiplexing mode.
 5. The method of claim 4, further comprising:providing the plurality of data streams for transmission from aplurality of antennas to the one user terminal.
 6. The method of claim1, wherein a plurality of user terminals are selected for datatransmission and the selected spatial multiplexing mode is a steeredspatial multiplexing mode.
 7. The method of claim 6, further comprising:spatially processing the plurality of data streams with a plurality ofsteering vectors to steer the plurality of data streams to the pluralityof user terminals.
 8. The method of claim 6, further comprising:performing receiver spatial processing on a plurality of received symbolstreams to obtain estimates of the plurality of data streams transmittedby the plurality of user terminals, wherein each data stream isprocessed with a respective steering vector to steer the data stream. 9.The method of claim 1, wherein a plurality of user terminals areselected for data transmission and the selected spatial multiplexingmode is a non-steered spatial multiplexing mode.
 10. The method of claim9, further comprising: performing receiver spatial processing on aplurality of received symbol streams to obtain estimates of theplurality of data streams transmitted by the plurality of userterminals.
 11. The method of claim 9, further comprising: providing theplurality of data streams for transmission from a plurality of antennasto the plurality of user terminals each having multiple antennas. 12.The method of claim 1, wherein the MIMO system is a time division duplex(TDD) system.
 13. The method of claim 12, wherein the selected spatialmultiplexing mode is a steered spatial multiplexing mode if the at leastone user terminal is calibrated and downlink channel response isreciprocal of uplink channel response.
 14. The method of claim 12,wherein the selected spatial multiplexing mode is a non-steered spatialmultiplexing mode if the at least one user terminal is uncalibrated anddownlink channel response is not reciprocal of uplink channel response.15. The method of claim 1, wherein the selecting a plurality of ratesincludes estimating signal-to-noise-and-interference ratios (SNRs) ofthe plurality of spatial channels, and selecting the plurality of ratesbased on the estimated SNRs of the plurality of spatial channels.
 16. Anapparatus in a wireless multiple-access multiple-input multiple-output(MIMO) communication system, comprising: a terminal selector operativeto select at least one user terminal for data transmission; a modeselector operative to select a spatial multiplexing mode, from among aplurality of spatial multiplexing modes supported by the system, to usefor the at least one user terminal; a rate selector operative to selecta plurality of rates for a plurality of data streams to be transmittedvia a plurality of spatial channels of a MIMO channel for the at leastone user terminal; and a scheduler operative to schedule the at leastone user terminal for data transmission with the plurality of selectedrates and the selected spatial multiplexing mode.
 17. The apparatus ofclaim 16, further comprising: a transmit spatial processor operative tospatially process the plurality of data streams in accordance with theselected spatial multiplexing mode to obtain a plurality of transmitsymbol streams for transmission from a plurality of antennas to the atleast one user terminal.
 18. The apparatus of claim 16, furthercomprising: a receive spatial processor operative to spatially process aplurality of received symbol streams in accordance with the selectedspatial multiplexing mode to obtain estimates of the plurality of datastreams transmitted by the at least one user terminal.
 19. An apparatusin a wireless multiple-access multiple-input multiple-output (MIMO)communication system, comprising: means for selecting at least one userterminal for data transmission; means for selecting a spatialmultiplexing mode, from among a plurality of spatial multiplexing modessupported by the system, to use for the at least one user terminal;means for selecting a plurality of rates for a plurality of data streamsto be transmitted via a plurality of spatial channels of a MIMO channelfor the at least one user terminal; and means for scheduling the atleast one user terminal for data transmission with the plurality ofselected rates and the selected spatial multiplexing mode.
 20. Theapparatus of claim 19, further comprising: means for spatiallyprocessing the plurality of data streams in accordance with the selectedspatial multiplexing mode to obtain a plurality of transmit symbolstreams for transmission from a plurality of antennas to the at leastone user terminal.
 21. The apparatus of claim 19, further comprising:means for spatially processing a plurality of received symbol streams inaccordance with the selected spatial multiplexing mode to obtainestimates of the plurality of data streams transmitted by the at leastone user terminal.